Plasma reactor for processing a transparent workpiece with backside process endpoint detection

Abstract

A plasma reactor is provided for processing a workpiece such as a transparent mask or a semiconductor wafer that is transparent at least within a range of wavelengths. The reactor includes a vacuum chamber having a sidewall and a ceiling. A workpiece support pedestal has a support surface facing said ceiling and lying within said chamber for supporting a workpiece. A passage extends through said workpiece support pedestal from a bottom thereof and forms an opening through said support surface. The reactor further includes an optical fiber extending through said passage. The optical fiber has: (a) a viewing end with a field of view through said opening in said support surface, and (b) an output end outside of said chamber. The reactor also includes an optical sensor coupled to said output end of said optical fiber which is responsive in said range of wavelengths.

1 . A plasma reactor for processing a workpiece that is transparent at least within a range of wavelengths, comprising:
 a vacuum chamber having a sidewall and a ceiling;   a workpiece support pedestal and having a support surface facing said ceiling and lying within said chamber for supporting a workpiece;   a passage extending through said workpiece support pedestal from a bottom thereof and forming an opening through said support surface;   an optical fiber extending through said passage and having: (a) a viewing end with a field of view through said opening in said support surface, and (b) an output end outside of said chamber; and   an optical sensor coupled to said output end of said optical fiber and being responsive in said range of wavelengths.   
   
   
       2 . The reactor of  claim 1  further comprising a lens in said passage at least near said support surface and having an optical axis extending through said opening in said support surface, said viewing end of said optical fiber facing said lens at or near said optical axis. 
   
   
       3 . The reactor of  claim 2  wherein said viewing end of said optical fiber is coupled to said lens at said optical axis. 
   
   
       4 . The reactor of  claim 2  further comprising:
 a light source having a spectrum that includes wavelengths within said range; and   a second optical fiber having one end lying outside of said chamber and coupled to receive light from said light source and another end coupled to said lens.   
   
   
       5 . The reactor of  claim 2  wherein said lens has sufficient power to resolve interference fringes generated in periodically spaced optical features of less than one micron in size on a workpiece supported on said workpiece support. 
   
   
       6 . The reactor of  claim 1  further comprising an optical signal processor coupled to said optical sensor. 
   
   
       7 . The reactor of  claim 6  wherein said optical sensor is capable of sensing an ambient reflected light level, and said optical signal processor is programmed to respond to a large shift in ambient reflected light level as being indicative of an etch process end point. 
   
   
       8 . The reactor of  claim 6  wherein said optical sensor is capable of sensing individual interference fringes, and said optical signal processor is programmed to count interference fringes generated on a workpiece supported on said pedestal during an etch process in said reactor. 
   
   
       9 . The reactor of  claim 6  wherein said optical sensor is a spectrometer, and said optical signal processor is programmed to compare a multiple wavelength interference spectrum with a known spectrum. 
   
   
       10 . The reactor of  claim 8  wherein said optical sensor is a spectrometer, and said optical signal processor is programmed to computed etch depth from spacing between spectral peaks in a spectrum produced by said spectrometer. 
   
   
       11 . The reactor of  claim 8  wherein said optical sensor is a spectrometer, and said optical signal processor is programmed to compare a multiple wavelength interference spectrum generated from said optical sensor with spectra of known etch depths in order to determine etch depth of a current process. 
   
   
       12 . The reactor of  claim 11  further comprising a memory accessible by said processor and storing said spectra of known etch depths. 
   
   
       13 . The reactor of  claim 8  wherein said optical sensor is an optical emission spectrometer with an operating range that includes said range of wavelengths, and said optical signal processor is programmed to track a selected spectral line for detecting etch process end point. 
   
   
       14 . The reactor of  claim 11  further comprising a light source having a spectrum that at least partially includes said range of wavelengths and a second optical fiber coupled between said lens and said light source. 
   
   
       15 . A method of monitoring the processing of a workpiece whose backside is held on a workpiece support pedestal in a plasma reactor, comprising:
 illuminating the backside of said workpiece with light furnished through said workpiece support, said light being of a wavelength range in which said workpiece is transparent;   viewing through said workpiece support reflected light from said workpiece.   
   
   
       16 . The method of  claim 15  wherein the step of viewing comprises sensing a shift in ambient reflected light level indicative of an etch process end point. 
   
   
       17 . The method of  claim 15  wherein the step of viewing comprises counting interference fringes to determine etch depth in said workpiece. 
   
   
       18 . The method of  claim 15  wherein the step of viewing comprises monitoring a multiple wavelength interference spectrum to determine etch depth in the workpiece. 
   
   
       19 . A method of processing a workpiece in a plasma reactor, comprising:
 monitoring light transmitted through the workpiece during processing in said reactor;   determining from changes in the light transmitted through the workpiece when a process end point has occurred.   
   
   
       20 . The method of  claim 19  wherein the step of monitoring comprises observing said light from a back side of said workpiece.